Heretofore, in the lithography technique, an exposure apparatus for manufacturing a semiconductor device by transferring a fine circuit pattern onto a wafer has been widely employed. Along with improving in high-integration, high-speed, and low power consumption of the semiconductor device have progressed, microsizing of the semiconductor device has advanced. In response to this trend, an exposure apparatus is required to image a fine circuit pattern of a semiconductor device on a wafer surface in a deeper focal length, and shortening in wavelength of the exposure light source has progressed. Specifically, for light used as the exposure light source, ultraviolet light of ArF excimer laser (wavelength of 193 nm) has been employed, further advancing form conventional g-line (wavelength of 436 nm), i-line (wavelength of 365 nm), or KrF excimer laser (wavelength of 248 nm). However, also in the lithography technique using such light with a wavelength of 193 nm, only semiconductor devices with circuit dimensions of 32 nm to 45 nm can be made, and development of a technique for making a semiconductor device with circuit dimensions of 30 nm or less is required. On the basis of such background, attention has been paid to the lithography technique using extreme ultraviolet light (EUV light) as a viable candidate, and development thereof has been actively performed. EUV light means light having a wavelength band in a soft X-ray area or a vacuum ultraviolet area, and specifically, light with a wavelength of about 0.2 nm to 100 nm. At the present point, use of light in the vicinity of 13.5 nm is mostly considered as the light source of lithography.
The exposure principle of the EUV lithography (hereinafter, abbreviated to “EUVL”) is identical with that of the conventional lithography in that a mask pattern is reduced-projected on a wafer by using a projection optical system, however, since there is no material through which light transmits in the EUV light energy region, a transmission refractive optical system using a transmission-type photomask which is generally used in an exposure apparatus having a light source of light with a wavelength of 193 nm to 436 nm cannot be used, and a reflecting optical system is used (refer to Patent Literature 1). An optical member of the reflecting optical system is constituted by a reflective photomask and a plurality of reflective mirrors, and is configured to reduced-project a pattern formed on the mask onto a resist formed on a wafer, to a rate of ¼ to ⅕ times thereof, through the reflective mirrors.
Herein, the reflective photomask is an optical member (EUVL optical member) obtained from, for example, a plurality of procedures described below.
First, a first procedure is to prepare a glass substrate which has a smooth and flat surface with extremely low roughness and without unevenness, as a base material for an optical member (optical member base material for EUVL). As such a glass substrate, it is required to have a low thermal expansion property so as not to cause expansion or contraction thereof by temperature changes even under EUV light irradiation, and hence, a substrate made of glass having a low thermal expansion coefficient (silica glass (SiO2 glass) or TiO2-containing silica glass (hereinafter, described as TiO2—SiO2 glass in the present specification) is generally used.
A second procedure is to prepare an ML blank in which a reflective layer reflecting EUV light is made on a surface of the glass substrate (film forming surface) on which a mask pattern is finally formed. As the reflective layer, a multilayered reflection film (ML film), in which high-refractive layers for EUV light (e.g., Si) and low-refractive layers (e.g., Mo) are stacked alternatively and thereby improving light reflectance at the time of irradiating the surface of the reflective layer with EUV light, is generally used. In addition, in order to prevent deterioration of the multilayered reflective film, a protective layer (e.g., Ru) is generally formed on the ML film.
Next, a third procedure is to form an absorber layer (e.g., Ta or TaN) that absorbs EUV light on the ML film of the ML blank (or on the protective layer when the protective layer has been formed). If necessary, an antireflection layer (e.g., TaON or TaO) having a low reflectance in a wavelength of mask pattern inspection light may be formed on the absorber layer.
It is a mask blank that obtained by forming an absorber layer and, if necessary, an antireflection layer on an ML blank.
A final fourth procedure is to form a mask pattern is formed by etching away a part of the absorber layer (or the absorber layer and the antireflection layer when the antireflection layer is formed on the absorber layer) of the mask blank, by using a resist or the like, so as to project on a wafer EUV light having desired light intensity distribution. It is a reflective photomask that has a portion at which a mask pattern is formed on the absorber layer (or the absorber layer and the antireflection layer when the antireflection layer is formed on the absorber layer) so that the reflective layer (or the protective layer when the protective layer is formed on the reflective layer) is exposed to reflect EUV light thereon thus obtained, and a portion at which the reflective layer (or the protective layer when the protective layer is formed on the reflective layer) is covered by the absorber layer (or the absorber layer and the antireflection layer when the antireflection layer is formed on the absorber layer) so that EUV light is hardly reflected thereon (refer to Non Patent Literature 1). Herein, a method for forming a mask pattern includes a series of steps of, for example, (1) forming a resist film on the absorber layer of the mask blank (or the absorber layer and the antireflection layer when the antireflection layer is formed on the absorber layer), (2) drawing a mask pattern on the resist film using a lithography device having a light source of an electron beam or ultraviolet ray in the state of holding the mask blank in a mechanical way such as clamping an end face or the vicinity of the outer circumference portion of the front and back surfaces of the mask blank, (3) removing the resist film in unnecessary portions, (4) etching the absorber layer (or the absorber layer and the antireflection layer when the antireflection layer is formed on the absorber layer) exposed due to the removal of the resist film, and (5) removing the remaining resist film.
In addition, a conductive film with sheet resistance of 100Ω or less (e.g., CrN, Cr, CrO, or TaN) is formed on a face where the mask pattern is not formed (hereinafter, referred as a “back surface”), in the case of a reflective photomask.
However, in an exposure apparatus using EUV light as the light source, a reflective photomask is adsorption-held by an electrostatic chuck by utilizing a conductive film which is formed on the back surface thereof, and a mask pattern that is formed on the film forming surface thereof is reduced-projected, and transferred onto a resist film on a wafer. At this moment, since all of the film forming surface and the back surface of the reflective photomask and the surface of the electrostatic chuck are not perfectly flat, the reflective photomask is adsorbed to the electrostatic chuck in the state of being deformed along the shape of the surface of the electrostatic chuck, substantially without causing a gap between the back surface of the reflective photomask and the surface of the electrostatic chuck. On the other hand, the mask pattern on the reflective photomask is formed by using a lithography device such as an electron beam or the like as described above, and the method for holding the reflective photomask in the lithography device is mechanical holding on the end face or the outer circumference, not by the electrostatic chuck. Thus, a mask pattern on the reflective photomask is formed in the state without deformation due to adsorption to the electrostatic chuck that occurs in an exposure apparatus. For this reason, if the reflective photomask is deformed at the time of being adsorbed to the electrostatic chuck in the exposure apparatus, the formation position of a pattern transferred onto a resist on a wafer is different from the formation position of a pattern on the reflective photomask formed using the lithography device. The pattern transferred on a resist on a wafer has to be satisfactorily and accurately copied from the pattern on the reflective photomask that is the original pattern, except for being reduced to ¼ to ⅕ times, which is the projection magnification of the exposure apparatus. Therefore, it is a problem that a deviation of the pattern formation position caused by deformation of the reflective photomask due to the electrostatic chuck adsorption in the exposure apparatus as described above fails to acquire intended transfer accuracy.
For this reason, it is preferable that the deformation of the reflective photomask when being adsorption-held by the electrostatic chuck in the exposure apparatus be as small as possible. For this, it is preferable that both of (1) flatness of the surface of the electrostatic chuck and (2) flatness of the film forming surface and back surface of the glass substrate be as low as possible. In addition, it is preferable, in the exposure apparatus, that the shape of the film forming surface of the reflective photomask being adsorption-held by the electrostatic chuck be the same as the shape of the film forming surface of the reflective photomask being held in the lithography device for forming a mask pattern. For this, it is necessary to extremely reduce both (3) total thickness distribution (generally, TTV) of the glass substrate and (4) the gap between the surface of the electrostatic chuck and the back surface of the reflective photomask. The requirement (4) is inevitably reduced if the requirements (1) and (2) become lower. For this reason, very strenuous demands are made on the requirement (1) of flatness of the surface of an electrostatic chuck to be 40 nm or less (Table 1 of Non Patent Literature 2), and both of the requirement (2) of flatness of the film forming surface and the back surface of the glass substrate and the requirement (3) of total thickness distribution of the glass substrate to be 100 nm or less, preferably 30 nm or less together (Table 4 of Non Patent Literature 3). Such strenuous demands for the glass substrate are specific for the reflective photomask held by using an electrostatic chuck during exposure, in other words, specific for a mask blank for EUVL.
As a method for processing a glass substrate for realizing such strenuous demands, as disclosed in Patent Literatures 2 to 4, various local polishing methods which are for selectively and locally polishing only part of the film forming surface and/or back surface of the glass substrate, have been proposed. In such methods, however, it was problematic in that both of the total thickness distribution of the glass substrate and flatness of the film forming surface and back surface of the glass substrate could not be certainly satisfied at the same time. In addition, Patent Literature 5 has proposed a method for obtaining a glass substrate being excellent in flatness and total thickness distribution by simultaneously polishing the entire substrate surface, which is called an entire surface polishing method. According to the proposal, by locally controlling a substrate pushing pressure during polishing, the speed of polishing is locally adjusted so as to obtain a desired surface profile, and therefore, a glass substrate having both of such excellent flatness and total thickness distribution that flatness of the film forming surface and back surface be 40 to 50 nm or less and total thickness distribution be 50 nm or less, is obtained.
However, even when the above-described proposed various methods are used, both demands of flatness and total thickness distribution of the glass substrate are very strict and it is extremely difficult to meet both demands at the same time, and thus, a method in which formation position of a mask pattern is adjusted on the basis of the flatness of the glass substrate, when a mask pattern is drawn by using an electron beam, or the like, on a reflective photomask, has been proposed (Non Patent Literatures 4 and 6). If this method is applied, the demand on flatness of the glass substrate is moderated to 300 nm or lower, and the demand on the total thickness distribution is not particularly required. However, the adjustment amount of pattern position during drawing a mask pattern depends on the total thickness distribution of the glass substrate, and the adjustment amount of pattern position during electron beam drawing may become smaller as the total thickness distribution of the glass substrate becomes smaller, and therefore, excellent transfer accuracy during implementation of EUVL is stably obtained. Specifically, in the case of the glass substrate having flatness of 300 nm described in Non Patent Literatures 4 and 5, the total thickness distribution thereof can be a very large value of 600 nm to the maximum. For this reason, even when flatness of the glass substrate is moderated from 100 nm or lower that is a conventional demand and becomes 300 nm or lower, it is preferable that the total thickness distribution of the glass substrate be as small as possible since the adjustment amount of pattern position during drawing a mask pattern may be small and excellent transfer accuracy during implementation of EUVL is stably obtained.
Furthermore, for the glass substrate, in addition to flatness of the film forming surface and back surface and total thickness distribution, as mentioned in Table 5 of Non Patent Literature 3, it is required to have no defects such as scratches or streaks of which the depth is 1 nm or more, and have no defect such as minute unevenness of which the size converted to a polystyrene latex particle diameter is 50 nm or larger on the film forming surface. Recently, it is required to have no serious defect, such that the influence of the defect on the glass substrate cannot be reduced by correction of a mask pattern, on the film forming surface. As a specific example, Non Patent Literature 5 reports that a defect having a height of 120 nm cannot be reduced its influence by correction of a mask pattern, and it is strongly required to have, on the surface, no concave defect such as a scratch, a streak, or a pit or convex defect such as foreign substances attached to the surface, having a relatively large size converted to a polystyrene latex particle diameter of 150 nm or larger.
In other words, it is required to provide a glass substrate, which has small flatness for the film forming surface and back surface, has small total thickness distribution, and has no remarkable scratch, streak, or the like, and of which a mask pattern can be corrected.